1. Field of the Invention
This invention relates to a semiconductor integrated circuit device allowing the change of the product specification and a chip screening method therewith.
2. Description of the Related Art
In dynamic RAMs (hereinafter, referred to as DRAMs), the ratio of the refresh time T to the refresh cycle R is T/R=15.6 .mu.sec. This ratio holds true for each generation of DRAMs: for example, 8 msec/512 cycles for the 1 Mbit DRAM generation, and 16 msec/1024 cycles (hereinafter, 1024 cycles are referred to as 1-kcycles) for the 4 Mbit DRAM generation.
For the 16 Mbit DRAM generation and later, the relation ship should be 32 msec/2048 cycles (hereinafter, 2048 cycles are referred to as 2-kcycles). To reduce power consumption, prevent heat generation, and make the active current smaller, however, it is necessary to increase the number of refresh cycles to decrease the number of cell arrays to be activated at the same time. For example, the number of refresh cycles is increased to 4096 cycles (hereinafter, referred to as 4 kcycles).
Additionally, there is a need to reduce the number of refresh cycles in order to manufacture multi-bit symmetrical address products. For example, 1-kcycles are used. To prevent the chip size from becoming larger and to ensure the sensitivity (C.sub.B /C.sub.S where C.sub.B is the bit-line capacity and C.sub.S is the cell capacity), the number of cells per bit-line cannot be changed from the present value (for example, 128 cells per bit-line), so that it is natural to change the number of refresh cycles.
Changing the number of refresh cycles means that the chip must be redesigned each time the number is changed. This imposes a heavy burden on the circuit designing personnel, resulting in reduced development efficiency.
More diversification of products requires factories to produce a variety of products simultaneously, reducing the production efficiency.
Additionally, the conventional chip screening test only rejects defective products. In this test, chips that fail to come up to the passing mark set for each product are judged to be unacceptable, and are discarded. Because of this, the conventional chip screening test has contributed to a poorer product yield.